1. Field of the Invention
The present invention relates to an apparatus for synthesizing a musical tone signal which has a loop circuit. More particularly, the present invention relates to an apparatus for synthesizing a musical tone signal in which a musical tone synthesizing method of a delay feed back type is employed so that, when a wave form signal is inputted into a loop circuit including a delay circuit, a circulated process is performed therein to synthesize a musical tone signal.
2. Prior Art
A digital tone generator including a loop circuit having a delay circuit have been provided in electronic musical instruments for synthesizing musical tone signals. The digital tone generator including the loop circuit is dependent on the so-called musical tone synthesizing algorithm of the delay feed back type in which a specified signal, such as an initial wave form signal, an impulse signal, or a nonlinear signal, is inputted into the loop circuit and a recurrence calculation process is carried out (Japanese patent publication numbers sho 58-58679, 58-48109, 59-7396, and Laid-open publication numbers sho 62-143097, 63-80300, 63-40199, and the like).
The apparatus of this type is called a physical model tone generator. Because, an actual mechanical or air vibration series is represented physically and approximately by an electronic circuit. For example, the Japanese Patent Laid-open publication number sho 63-40199 discloses the art which includes a loop circuit having a delay element for simulating a string tone. An excitation signal to be outputted to the loop circuit is generated based on a signal from the loop circuit and a signal corresponding to a bow velocity and a bow pressure of a natural string instrument.
However, the above type tone generator requires accurate control of playing information to obtain stable, natural feeling musical tones, because the tone generator is arranged so as to simulate a tone generation principle of the natural string instrument. In addition, the control requires the same technique as the playing of the natural string instrument. Therefore, the control and the playing operation of the electronic musical instrument having the above type tone generator causes complicated control and playing operation.
FIG. 7 shows a conventional musical tone synthesizing series of a model which simulates very much the vibration series of the natural string instrument. In this strict model, it is difficult to obtain any desired tone pitch. A pitch frequency depends on the reciprocal of the sum of the delay amount of two delay circuits 21a, 21b and two filters 22a, 22b. But the accurate value of the pitch frequency isn't seen unless the musical tone is actually synthesized. The actual pitch frequency is often slightly lower than that which the above mentioned linear series decides. Also, in this accurate model, a strange musical tone is often generated, and the musical tone isn't outputted at all with an audio frequency though that has to be outputted under a certain condition. This way, the pure physical model of the musical tone synthesizing series has a problem that the control is very difficult. The way of modifying the loop circuit configuration or limiting input signals, such as the signals corresponding to the bow velocity and the bow pressure, within a specified range is capable of causing easy control and playing operation. However, the outputted musical tone is far from the natural one and tone color varying is monotonous, and therefore it is impossible to obtain natural feeling musical tones continuously.
Still more, if a memory device is provided to store as much wave form data as the natural instrument generates, a very large memory device is required.